Semiconductor devices are found in many products in the fields of entertainment, communications, networks, computers, and household markets. Semiconductor devices are also found in military, aviation, automotive, industrial controllers, and office equipment. The semiconductor devices perform a variety of electrical functions necessary for each of these applications.
Semiconductor devices operate by exploiting the electrical properties of semiconductor materials. Generally, semiconductor materials have electrical properties that vary between those of conductors and insulators. In most cases, semiconductors have poor electrical conductivity, however their conductivity can be modified through the use of doping and/or applied electrical fields. Doping involves introducing impurities into the semiconductor material to adjust its electrical properties. Depending on the amount of doping performed, semiconductor materials may be permanently modified to conduct electricity as well as other conductors or to act as insulators. The application of electric fields also modifies the conductivity of semiconductor materials by modifying the distribution of conductive particles within the material. Through doping and the application of electronic fields, integrated circuits are formed and operated over a semiconductor substrate. The circuits include multiple layers of semiconductor, insulator and conductive materials.
Because the electrical properties of semiconductor materials may be altered by the application of electric fields, they can be used to manufacture both passive and active circuit elements. Passive circuit elements include capacitors, inductors, resistors and other circuit elements that are not capable of power gain. Active circuit elements, however, include transistors and allow for the creation of circuits that can both amplify and switch electrical signals. Transistors are the fundamental elements of modern computing systems and allow for the formation of logic circuits that include complex functionality and provide high performance.
Many transistors can be combined into a single integrated circuit formed over a semiconductor wafer or substrate. Integrated circuits combine many transistors and other passive and active circuit elements over a single substrate to provide complex electronic circuits such as processors, microcontrollers, digital signal processors, and memory systems. Modern integrated circuits may include tens of millions of transistors and provide the complex functionality of all computing systems. Integrated circuits and other semiconductor devices in electronic systems provide high performance in a small area and may be created using cost-efficient manufacturing processes.
The manufacture of semiconductor devices and integrated circuits involves formation of a wafer having a plurality of die. Each semiconductor die contains transistors and other active and passive circuit elements performing a variety of electrical functions. For a given wafer, each die from the wafer typically performs the same electrical function. Semiconductor devices are formed in two steps referred to as front-end and back-end manufacturing that involve formation of the die and packaging for an end user.
Front-end manufacturing generally refers to formation of the semiconductor devices on the wafer. During formation of the devices, layers of a dielectric material such as silicon dioxide are deposited over the wafer. The dielectric facilitates the formation of transistors and memory circuits. Metal layers are deposited over the wafer and patterned to interconnect the various semiconductor devices. The finished wafer has an active side containing the transistors and other active and passive components. After the devices are formed, they are tested in a preliminary testing step to verify the devices are operational. If a sufficiently high number of devices are discovered to contain defects, the devices or even the entire wafer may be discarded.
Back-end manufacturing refers to cutting or singulating the finished wafer into the individual die and then packaging the die for structural support and environmental isolation. To singulate the die, the wafer is scored and broken along non-functional regions of the wafer called saw streets or scribes. In some cases, the wafer is singulated using a laser cutting device. After singulation, the individual dies are mounted to a package substrate that includes pins or contact pads for interconnection with other system components. Contact pads formed over the semiconductor die are then connected to contact pads within the package. Often, wire bonding is used to make the connection, however other connection technologies such as solder bumps or stud bumping may be used. After wire bonding, an encapsulant or other molding material is deposited over the package to provide physical support and electrical insulation. The finished package is then inserted into an electrical system and the functionality of the semiconductor device is made available to the other system components.
One goal of semiconductor manufacturing is to produce a package suitable for faster, reliable, smaller, and higher-density integrated circuits at lower cost. Flip chip packages or wafer level packages are ideally suited for integrated circuits demanding high speed, high density, and greater pin count. Flip chip style packaging involves mounting the active side of the die face down toward a chip carrier substrate or printed circuit board (PCB). The electrical and mechanical interconnect between the circuit elements on the die and conduction tracks on the carrier substrate is achieved through a solder bump structure comprising a large number of conductive solder bumps or balls. The solder bumps are formed by a reflow process applied to solder material deposited on contact pads which are disposed on the semiconductor substrate. The solder bumps are then soldered to the carrier substrate. The flip chip semiconductor package provides a short electrical conduction path from the active circuit elements on the die to the carrier substrate in order to reduce signal propagation distance, lower capacitance, and achieve overall better circuit performance.
Often the semiconductor package includes dies having common voltage buses. The buses include conductive materials such as copper formed over a relatively large area. Although the buses allow for several die interconnections having the same common voltage, the large area of thick conductive material generates high levels of residue stress. The stress can cause damage to the IC active circuitry and general package reliability problems.
In conventional packages, contact pads are formed over the package for electrical interconnect. Often the interconnect is formed using wirebonds connected to contact pads formed over the package. The formation of conventional wirebond interconnect structures requires high-cost front end wafer processing including chemical mechanical polishing (CMP), chemical vapor deposition (CVD) and reactive ion etching (RIE) to form the rerouted peripheral input/output pads for wire bonding. The formation of peripheral input/output pads is expensive and results in a package having a relatively large footprint.